Russian Foreign Minister Sergey Lavrov has portrayed his country's airstrikes in Syria on Thursday as limited amid reports of heavy casualties in the eastern part of the country.

Mr Lavrov told reporters at an economic summit in Peru that Russian and Syrian air forces are only working together to prevent Islamic State fighters who are fleeing the Iraqi city of Mosul from crossing into Syria.

The Russian politician spoke after a private meeting with US Secretary of State John Kerry during the Asia Pacific Economic Cooperation Forum in Lima.

Mr Kerry said they spoke about "every aspect" of Aleppo, among other topics that included Libya and Yemen.

They were asked about reports of air strikes in Aleppo.

"Our air force and the Syrian air force work only in the provinces of Idlib and Homs to prevent the Isil who might be leaving Mosul from getting to Syria," Mr Lavrov said.

Syrian activists and rescue workers said airstrikes in Aleppo killed more than 20 people on Thursday amid a major Syrian and Russian offensive in opposition-held areas of Syria.